Subterranean Preparation

Prior to launch, the agency spent three years—three times as long as the tunneling work—in design and preparation, including relocating utilities. In the process, contractors discovered Indian midden—remains of shells deposited along with tools.

Removal of the midden find followed. "Luckily this happened before the TBMs were in place so the work stoppage was less impactful," says Funghi.

He estimates excavation and transfer to Sonoma State University some 60 miles away took six to nine months. "Whenever you work with 100-year-old streets you find surprises," Funghi says.

Tricamo says the most challenging section of tunneling was under the Bay Area Rapid Transit subway line that moves 500,000 people daily from the suburbs, across the bay, through the city and to South San Francisco and back. The new tunnels come within 10 ft vertically of the BART line.

At the "crisscross" location, the design called for a 450-ft-long turn in the tunnel. That can sometimes result in over-excavation, says Fowler.

Active articulation allowed the TBM to make the turn by transferring pressure equally to the circumference of the ring between the front and rear section, he adds. The contractor reduced the speed and used a continuous, stable flow of material.

"We lost a little production to ensure smooth progress," says Tricamo.

The contractor never had to employ the surface compensation grout ports installed at 2-ft increments as a safety precaution. The team worked in shifts 24 hours per day, every day, through this area rather than the typical two, 10-hour shifts per day.

A community review panel monitored the active line and the surface for any impact. "No one felt anything," says Funghi. He attributes the lack of jarring to the hydraulic jack thrust rams that propel the TBM by pushing off completed segments without motor vibration.

Subcontractor Wang Technology installed more than 2,000 sensors—electrolevel tiltmeters, inclinometers, piezometers, multipoint borehole extensometers, strain gauge arrays and grid-crack gauges—on surrounding buildings and roadways and in the BART subway.

Technicians could measure ground movement and relay the information in real time to computers in SFMTA offices, to smartphones on site and to the cabin in the TBM where 10 touch-screen monitors allow operators to make incremental changes to preprogrammed instructions.